(1) Field of the Invention
The present invention relates to a device for supplying current to at least one silicon rod during precipitation of silicon according to the Siemens process. The invention further relates to a method for supplying current to at least one silicon rod during precipitation of silicon according to the Siemens process using the device.
(2) Description of Related Art
Devices for supplying current to a silicon rod during precipitation of silicon according to the Siemens process are known in the art. Such devices have an input for connecting the device to an electric power grid for supplying electric energy. The devices further have at least one output for connection to one or several silicon rods. One or several AC current regulators supply electric current from the electric power grid to the silicon rod(s) connected to the output(s). The current intensity and the voltage and hence also the electric power thermally converted in the silicon rod can be adjusted with the AC current regulator.
When producing silicon rods with the Siemens process, these devices are used to supply current to the silicon rods located inside a reactor. The reactor is filled with trichlorosilane and is under pressure. The power thermally dissipated in the silicon rods as a result of the current flow causes the silicon rods to heat up. The surface temperature of the silicon rods reaches about 1100° C., at which temperature trichlorosilane decomposes and dissociates to silicon and hydrogen chloride under addition of hydrogen. The silicon precipitates on the silicon rod(s) arranged in the reactor. The precipitation process causes an increase in the diameter of the silicon rod during the process by about 1 mm/hour. The cross-sectional area of the silicon rod increases accordingly. With a constant specific resistance of the silicon rod, the larger cross-sectional area causes the electrical resistance of the rod to decrease. In addition, the current density changes from a quantity that is constant along the radius to a quantity that strongly depends on the radius. The resistance is independent of the radius for diameters of the silicon rods of 7-10 mm. At larger diameters, the current density is greater in the interior of the rod than at the periphery.
Because the electrical conductivity of silicon increases with a temperature while the thermal conductivity of silicon decreases with temperature, constriction of the current at the center of the rod results in a temperature gradient from the inside to the outside. The temperature gradient can be so extreme that the rod reaches temperatures of greater than 1460° C. in the interior, whereas the temperature at the outside periphery of the silicon rod is 1100° C. However, a temperature of 1460° C. at the core of the silicon rod causes the silicon in the core of the rod to melt. The silicon can then flow out of the rod, destroying the rod. The precipitation temperature therefore has to be adjusted so that the core temperature does not exceed the melting temperature of silicon.
However, the precipitation temperature should be as high as possible so as to attain a large growth rate of the rod and/or a high precipitation velocity.
The published international patent application WO 97/36822 describes a device for precipitating silicon on silicon rods with the Siemens process. The published document also addresses the problem due to the temperature gradient between the core and the outer surface of the silicon rod. The document proposes to install an additional heater which generates eddy currents in the silicon rod. The eddy currents generate a magnetic AC field with a predetermined frequency, which causes the current density to change. The current density becomes more homogeneous from the peripheral surface of the silicon rod toward the core of the silicon rod, thus producing a more uniform temperature distribution.
The device proposed in the publication is complex, because a second heater, namely the eddy current heater, must be provided in addition to the conventional resistance heater for heating the silicon rods.
The published German patent document DE 198 82 833 T1 discloses a device which takes advantage of the skin effect produced in a conductor at high or medium frequency currents in order to attain a uniform temperature distribution inside a silicon rod. The disclosed device has a single heater, namely a resistive heater. This resistive heater, however, is operated with currents having different frequencies. According to the technical teachings of this document, a low-frequency voltage is initially applied to the silicon rod. When the temperature of the silicon rod exceeds a predetermined value, the low-frequency voltage is switched off and a voltage with a higher frequency is applied. However, the document DE 198 82 833 T1 does not describe in detail a corresponding device for supplying electric energy to the silicon rod.
The method disclosed in the document DE 198 82 833 T1, namely to supply the silicon rods either only with low-frequency currents or only with high or medium frequency currents, disadvantageously generates relatively high inductive losses when only a high or medium frequency voltage is supplied to the silicon rod.